At present, most matrix based display technologies are in its technological infancy compared to long established electronic image forming technologies such as Cathode Ray Tubes. As a result, many domains of image quality deficiency still exist and cause problems for the acceptance of these technologies in certain applications.
Matrix based or matrix addressed displays are composed of individual image forming elements, called pixels (Picture Elements), that can be driven (or addressed) individually by proper driving electronics. The driving signals can switch a pixel to a first state, the on-state (luminance emitted, transmitted or reflected), to a second state, the off-state (no luminance emitted, transmitted or reflected), see for example EP-117335 which describes an LCD. For some displays, one intermediate state between the first and the second state is used—see EP-462619 which also describes an LCD. For still other displays, one or more intermediate states between the first and the second state (modulation of the amount of luminance emitted, transmitted or reflected) are used—see EP-117335.
Matrix addressed displays are typically composed of many millions of pixels and very often pixels exist that are stuck in a certain state (on, off or anything in between). Where sub-pixel elements are individually controllable then one of the sub-pixel elements may become stuck in a certain state. For example, a pixel structure may comprise three sub-pixel elements for red, green and blue colours. If one of these sub-pixels becomes stuck, then the pixel structure has a permanent colour shift. Mostly such problems are due to a malfunction in the driving electronics of the individual pixel (for instance a defect transistor). Other possible causes are problems with various production processes involved in the manufacturing of the displays, and/or by the physical construction of these displays, each of them being different dependent on the type of technology of the electronic display under consideration. It is also possible that a pixel or sub-pixel element is not really stuck in a state, but shows a luminance or colour behaviour that is significantly different from the pixels or sub-pixels in its neighbourhood. For instance but not limited to a defective pixel shows a luminance behaviour that differs more than 20% (at one or more video-levels) from the pixels in its neighbourhood, or a defective pixel shows a dynamic range (maximum luminance/minimum luminance) that differs more than 15% from the dynamic range of pixels in its neighbourhood, or a defective pixel shows a colour shift greater than a certain value comparing to an average or desired value for the display. Of course other rules are possible to determine if a pixel or sub-pixel is defective or not (any condition that has a potential danger for image misinterpretation can be expressed in a rule to determine whether a pixel is a defective pixel). The exact reason for a defective pixel is not important for the present invention.
Defective pixels are very visible for a user of the display. This not only can be very disturbing for the user, but it can also result in wrong interpretation of the image being displayed. For applications where image fidelity is required to be high, such as for example in medical applications, this situation is unacceptable.
U.S. Pat. No. 5,504,504 describes a method to make some classes of defective pixels less visible by changing the luminance of pixels in the neighbourhood of the defective pixel. This, however, distorts the luminance of the whole image, and is not generally accepted in the medical world when viewing medical images.
In prior art devices, some classes of defective pixels are located and burnt down so as to always appear black and thus less visible to a user. This solution, however, has the disadvantage that, e.g. in medical images, a black pixel in a region of interest for a radiologist might indicate (or hide) for example a pathological defect such as tumour cells, Therefore, there is a danger in such a situation that a radiologist will not be able to discriminate between the defective pixel and a feature of therapeutic relevance.